Interaction energies for the binding of three intercalators to nucleic acid base pairs and base-pair steps are presented. Density fitting (DF) and local (L) correlation methods are employed, allowing use of basis sets appropriate for description of non-covalent interactions. In common with previous studies of stacking interactions, DF-LMP2 overestimates binding by as much as 50%. However, spin-component scaling (SCS) corrects for this effect, resulting in binding energies that support literature data obtained with small basis sets and/or density functional theory. The efficiency of this approach allows intercalators within base-pair steps to be studied, revealing substantial many body terms. (c) 2009 Elsevier B.V. All rights reserved.